1. Field of the Invention
The present invention relates to copolymers. The present invention also relates to organic electroluminescent devices, which are employed in the field of display devices or the like, and in which a copolymer useful in hole transport layers is used. Furthermore, the present invention relates to a method of producing hole transport layers for organic electroluminescent devices.
2. Description of Related Art
Organic electroluminescent devices are spontaneous light-emitting devices which convert excitation energy, which is generated when holes and electrons injected from an anode and a cathode, respectively, travel in an organic layer and are recombined in an organic fluorescent material, into emission energy. This type of spontaneous light-emitting device is of major interest for large area display devices driven by low voltages.
A device structure which is layered with organic layers having different carrier transport properties effectively enhances the efficiency of organic electroluminescent devices and, for example, it has been reported that a device comprising a hole transport layer made of a low-molecular-weight aromatic amine and an electron-transporting emitter layer made of an aluminum chelete complex, which are formed by a vacuum deposition method, has sufficiently high luminance of 1000 cd/m2 suitable for practical use with an applied voltage of 10 V or less (see, for example, “Applied Physics Letters”, 1987, Vol. 51, p. 913, and U.S. Pat. No. 4,356,429). Subsequently, extensive reseach has been performed on this type of multi-layered organic electroluminescent device.
However, as research progressed, various problems in multi-layered organic electroluminescent devices became apparent. While there has been much research on the effects of hole transport layers on characteristics of electroluminescent devices, it has been pointed out that there are problems with organic electroluminescent devices using a low-molecular-weight hole transport material in that the devices are degraded by heat generated during current injection as well as by crystallization or agglomeration in organic layers over time, and in that the emission efficiency is reduced due to formation of exciplexes when mutual diffusion occurs at the interface between a hole transport layer and an emitter layer. These problems were solved by using a hole transport layer made of a high-molecular-weight hole transport material (see, for example, “Trends in Polymer Science”, 1994, Vol. 2, p. 350, and Japanese Unexamined Patent Application, First Publication No. Hei 9-255774).
However, since a conventional high-molecular-weight hole transport material does not have reactive functional groups and cannot form chemical bonds in the direction in which the layers are laminated, it is impossible to enhance adhesion of the hole transport layer with another layer, particularly with an anode surface, and it is impossible to achieve light emission having high luminance with low applied voltages. Even if an excellent hole transport material were available, the hole transport material would not be usable if the solubility of the material in a solvent used to form the hole transport layer and the solubility of the material in a solvent used to form another organic layer are the same.